1. Field of the Invention
This invention describes a new solution for obtaining user controlled high reflection factors using wideband pre-matching microwave tuners.
2. Description of the Prior Art
High reflection factors are obtained actually using either permanent pre-matching networks on the RF circuits or test fixtures themselves (FIG. 1) or/and pre-matching tuners (FIG. 2). In more detail, the problem of high reflection factor microwave tuning, required for Load Pull and Noise testing of high power and low noise transistors, using automatic or manual microwave tuners, has been addressed up to date in different manners as follows:
1. Using (Pre-matching) Transforming Networks (FIG. 1)
These transforming networks (3, 7) are manufactured on the test fixtures (6) or ‘on-chip’ meaning as integral part of the semiconductor circuit (monolithic integrated circuit). This allows a static pre-matching, in which the characteristic impedance of the test system (typically 50 Ohm) is transformed to values, which lie closer to the conjugate complex of the internal impedance of the DUT (device under test) (1). This technique makes further tuning by external controllable automatic or manual tuners, connected to the ports (5, 9) of the test fixture (6) easier and more accurate.
This static pre-matching technique has been used for long time in RF technology, but has the disadvantage of not being able to cover a significant frequency bandwidth or match a variety of DUT's (1) using the same transforming networks (3, 7), which said networks must be re-designed and manufactured for every other test frequency and DUT.
2. Using Pre-matching Tuners (FIG. 2)
Pre-matching tuners consist of a housing (10), a slotted airline or slabline (21), and two mobile carriages (12,13), which are driven by two lateral mechanisms such as driving screws (17) and (16), which themselves are controlled by stepping motors via electrical signals (14,15). Each carriage has a vertical axis, which can insert or withdraw RF probes (slugs) (18, 19) into the slotted airline (21) again driven by electrical stepper motors.
These devices have the capability of generating very high reflection factors using the two (or more) RF slugs (18,19) in series and positioning them in such a manner that a first RF slug (pre-matching slug, 18) is positioned inside the airline (21) such as to generate a reflection factor (81, FIG. 3) close to the conjugate complex of the DUT's internal impedance. Then the second RF slug (tuning slug, 19) can tune easier and more accurately around the DUT's conjugate complex internal impedance (85), on a circle (84).
The theory and the experimental behavior behind this approach is basically the same as in case 1, i.e. transforming the characteristic impedance of the test system close to the conjugate complex internal impedance of the DUT before proceeding to the actual tuning. In comparison to case 1, this second method has the advantage of being adjustable i.e. by adjusting the position and depth of the first RF slug (18), we can determine the actual amplitude and angle of the peak of the pre-matching vector (81 or 82) to be close to the conjugate complex internal impedance of the DUT on the Smith Chart (80), and to be adjustable to match the internal impedance of various DUT's at various frequencies and other test conditions, without having to re-design the pre-matching networks, as is the case when using the technique of case 1 (FIG. 1). The second RF slug (tuning slug) can then be adjusted to reach maximum values (85, 88) to match even very high reflection DUT's.
Pre-matching tuners designed as shown in FIG. 2, i.e., incorporating two fully independent tuning sections (16,17) with associated tuner carriages (12,13) and RF slugs (19,18) have a serious disadvantage at higher frequencies, starting in fact around 5–6 GHz. The disadvantage is that the airline section between the position of the pre-matching slug (18) and the tuning slug (19) cannot be shorter than one half of the total length of the tuner itself (10), or one half of a wavelength at the lowest frequency of operation of the tuner, thus causing unnecessary insertion loss between the pre-matching and the tuning section, which reduces significantly the high reflection tuning capability of these pre-matching tuners, especially at higher frequencies. This minimum distance of one half of a wavelength (λ/2) at the minimum frequency of operation is necessary in order to allow a full 360° independent rotation of each of the pre-matching and tuning reflection vectors.
This fundamental characteristic of existing pre-matching tuners is due to basic design and mechanical restrictions related to the way those tuners operate, i.e. the fact that they require a permanent mechanical zero position as a fixed reference starting point for both the pre-matching as well as the tuning section of the tuner. Since tuners of this kind cover typically a wide frequency range of more than a decade (Max Frequency/Min Frequency>10), the electrical length of the idle and unused transmission line section between pre-matching and tuning RF sections corresponds to one half of a wavelength (λ/2) at the minimum frequency but to more than ten wavelength halves (or five full wavelengths) at the maximum frequency.
It is well known that insertion loss in transmission lines and in general in electromagnetic wave transmission is proportional to the total electrical length of the transmission section, amounting to a number of “Decibel (dB) per wavelength” at any given frequency. In other words, the insertion loss of a transmission line as used in RF tuners increases typically at least linearly, or more, with frequency.
The fixed minimum distance between the RF slugs leaves a section of transmission airline unused, who's relative length multiples of a wavelength (n*λ) increases as the frequency of operation increases (and the corresponding wavelength λ [mm]=300/frequency[GHz]), since at higher frequencies only a small part of the pre-matching section is used. This unused section of transmission airline contributes insertion loss between the first (pre-matching) section and the second (tuning) section of the pre-matching tuner. The effect of this insertion loss is such that it effectively jeopardizes the high reflection, otherwise possible, from this tuner structure at frequencies higher than the minimum frequency of operation of the tuner; this negative impact on the high reflection factor tuning capability of pre-matching tuners is even more predominant at very high frequencies and worsens as the frequency of operation increases, obviously because the number of idle wavelengths of the transmission line between pre-matching and tuning sections increases.
The tuning mechanism of a traditional pre-matching tuner made according to prior art is shown in FIG. 3. Tuning at lower frequencies (below 5–6 GHz) is represented with reflection vectors (81) and (83) on a circle (84). Tuning at higher frequencies, like 12 GHz to 40 GHz is shown in FIG. 3 with reflection vectors (82) and (86) on circle (87). Because of higher losses in the tuner connectors (22, FIG. 2) the pre-matching vector (82) is slightly smaller than the pre-matching vector (81). However the tuning vector (86) at high frequencies is considerably smaller than the tuning vector (81) at lower frequencies, with the final effect that the maximum reflection factor obtained at low frequencies (85) to be sensibly larger that the maximum reflection vector (88) at higher frequencies.
This is a considerable limitation for high frequency applications of automatic pre-matching tuners. In the presently used structures of automatic pre-matching tuners a solid separation in form of a physical wall or other fixed obstacle (20) between the tuning and the pre-matching section is inherently necessary in order to establish a fixed zero position for the tuning section and is used as starting point for tuner calibration and operation.
In conclusion, the idle unused transmission line length between the pre-matching and the tuning RF sections prevents existing pre-matching tuners from realizing their full potential in highest reflection factor, because the tuning vector overlapping with the pre-matching vector, is attenuated by the insertion loss of this idle transmission line section.